When amplifying power by using an amplifier such as a high power amplifier (hereinafter referred to as “HPA”) or the like, desired input-output characteristics may not be obtained due to nonlinear distortion characteristics of the amplifier.
Particularly when the frequency of a radio signal to be amplified is high, in order to linearize the amplifier by compensating the nonlinear characteristics, a complex IQ baseband signal of a low frequency, before being converted to the radio signal, needs to be subjected to predistortion for canceling the nonlinear distortion characteristics of the amplifier in advance by using digital signal processing, as described in Patent Literature 1.
In the predistortion process, a model or an inverse mode (distortion compensation model) of the amplifier is estimated, and a distortion in the amplifier is compensated based on the estimated model.
With increase in communication speed in recent years, it has been needed to amplify a wideband signal. When amplifying a wideband signal, since an output signal from the amplifier is distorted also by influence of a memory effect of the amplifier, such a distortion should be compensated. Non-Patent Literature 1 proposes a distortion compensation model of an amplifier taking into account such a memory effect.
As shown in FIG. 10, in an amplifier circuit having a distortion compensation section 101 for an amplifier 100, a conventional model taking into account a memory effect is expressed by equation (1).
                              [                      Equation            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                y          ⁡                      [            n            ]                          =                              ∑                          l              =                              -                                  L                  1                                                                    L              2                                ⁢                                    ∑                              k                =                1                                            K                l                                      ⁢                                          h                                  k                  ,                  l                                            ·                                                                                      u                    ⁡                                          [                                              n                        -                        l                                            ]                                                                                                          k                  -                  1                                            ·                              u                ⁡                                  [                                      n                    -                    l                                    ]                                                                                        (        1        )            
where
y[n] is an output signal of the amplifier 100,
k is an order,
l is a relative delay with respect to an input signal u[n] of the amplifier 100,
L1 is a maximum value of relative number of preceding samples,
L2 is a maximum value of relative number of delay samples,
K1 is a maximum order of the characteristic of the amplifier, wherein the index is a coefficient relating to the relative delay l, and
hk,l is a complex coefficient representing the characteristic of the amplifier 100, wherein the indices are a coefficient relating to the relative delay l, and a coefficient relating to the order k.
As shown in FIG. 11, in the conventional model expressed by equation (1), the memory effect that occurs inside the amplifier 100 is represented as a combination of a plurality of nonlinear elements NL (from memory term 0 to memory term L1+L2) having temporally different characteristics.
The characteristic of each of the plurality of nonlinear elements NL is expressed by equation (2).
As shown in equation (2), the nonlinear characteristic (input-output characteristic) of each nonlinear element NL is defined based on an input signal u[n′] to the amplifier 100.
                              [                      Equation            ⁢                                                  ⁢            2                    ]                ⁢                                                                                                            Y                          l              ′                                ⁡                      [                          n              ′                        ]                          =                              ∑                          k              =              1                                      K                                                l                  ′                                -                                  L                  1                                                              ⁢                                    h                              k                ,                                                      l                    ′                                    -                                      L                    1                                                                        ·                                                                            u                  ⁡                                      [                                                                  n                        ′                                            -                                              l                        ′                                                              ]                                                                                              k                -                1                                      ·                          u              ⁡                              [                                                      n                    ′                                    -                                      l                    ′                                                  ]                                                                        (        2        )            
where
Yl′[n′] is an output of the nonlinear element NL,
k is an order,
l′ is a relative delay with respect to the input signal u[n′],
L1 is a maximum value of relative number of preceding samples,
L2 is a maximum value of relative number of delay samples,
Kl′−L1 is a maximum order of the characteristic of the nonlinear element, wherein the index is a coefficient relating to the relative delay l′−L1, and
hk,l′−L1 is a complex coefficient representing the characteristic of the nonlinear element NL, wherein the indices are a coefficient relating to the relative delay l′−L1, and a coefficient relating to the order k.
Further, as shown in FIG. 11, each nonlinear element NL is given the input signal u[n′]. However, a delay element D is provided in a stage preceding each of the nonlinear elements NL corresponding to the memory terms 1 to L1+L2, so that the input signals u[n′] given to the respective nonlinear elements NL are temporally different from each other.
That is, the nonlinear elements NL corresponding to the memory terms 1 to L1+L2 represent the memory effect of the amplifier.
Accordingly, based on equation (2) and FIG. 11, the following equations (3a) and (3b) are derived as equations expressing models taking into account the memory effect.
                              [                      Equation            ⁢                                                  ⁢            3                    ]                ⁢                                                                                                y          ⁡                      [                                          n                ′                            -                              L                1                                      ]                          =                              ∑                                          l                ′                            =              0                                                      L                1                            +                              L                2                                              ⁢                                    ∑                              k                =                1                                            K                                                      l                    ′                                    -                                      L                    1                                                                        ⁢                                          h                                  k                  ,                                                            l                      ′                                        -                                          L                      1                                                                                  ·                                                                                      u                    ⁡                                          [                                                                        n                          ′                                                -                                                  l                          ′                                                                    ]                                                                                                          k                  -                  1                                            ·                              u                ⁡                                  [                                                            n                      ′                                        -                                          l                      ′                                                        ]                                                                                        (                  3          ⁢          a                )                                          y          ⁡                      [            n            ]                          =                              ∑                          l              =                              -                                  L                  1                                                                    L              2                                ⁢                                    ∑                              k                =                1                                            K                l                                      ⁢                                          h                                  k                  ,                  l                                            ·                                                                                      u                    ⁡                                          [                                              n                        -                        l                                            ]                                                                                                          k                  -                  1                                            ·                              u                ⁡                                  [                                      n                    -                    l                                    ]                                                                                        (                  3          ⁢          b                )            
Note that equation (3a) expresses a model directly derived from equation (2) and FIG. 11, and equation (3b) expresses a model derived by replacing n′−L1 and l′−L1 in equation (3a) with n and l, respectively.
Meanwhile, in order to enhance the power efficiency of the amplifier at the same time, a method has been proposed in which the power supply voltage (drain signal) of the amplifier is modulated by using the input signal of the amplifier, and the power consumption of the amplifier is dynamically varied in accordance with the magnitude of the input signal (this method is called “power supply modulation” or “envelope tracking”) (for example, refer to Patent Literature 2, and Non-Patent Literatures 2 and 3). In the power supply modulation method, when the voltage of the input signal is small, the power consumption of the amplifier is suppressed, whereby the power efficiency is enhanced. In this way, a high-efficiency amplification technique is provided.